In prior systems, access to information or to services is authorized only on presentation of a secret code, which code is compared directly or otherwise with a secret code recorded in a non-volatile memory of the memory card.
In most general terms, a card comprises one or more ICs having circuits etched thereon to form both a storage device and a processor member constituted by a microprocessor or by an equivalent electronic device (e.g. hard-wired logic). The storage device generally includes ROM or EEPROM type non-volatile memory containing the microprograms required to operate the card and/or a RAM type working memory for storing data and instructions specific to each application.
A special memory zone is conventionally provided in the non-volatile EEPROM memory for use in monitoring the presentation of secret codes. In known embodiments, the operating program of the microprocessor has the task of writing one or more bits in said memory zone each time a secret code is presented, regardless of whether it is presented correctly or not, thereby making it possible to prevent additional secret codes being presented after a determined number of wrong presentations. For example, in one known embodiment, the memory zone reserved for ensuring security during secret code presentation is divided into two zones: a first zone has one bit written therein on each successful attempt at presenting a secret code; and a second zone has one bit written therein on each unsuccessful attempt at presenting a secret code. This solution has the advantage of drawing an identical amount of current regardless of whether the attempt is successful or not, thereby preventing a fraudulent user obtaining information as to whether attempts are successful or otherwise on each occasion that an access code is keyed into the card, merely by observing the current drawn by the card.
However, this solution suffers from several drawbacks including firstly occupying a considerable amount of data storage space, and secondly wasting additional data storage space for a card that possesses a plurality of secret codes each associated with its own zone.
Finally, when the secret code presentation zones are saturated, the secret code associated with the card can no longer be used and the card must be thrown away.
The object of the invention is to mitigate the above drawbacks.